Through the candidate gene approach, we have identified MYH9 as the causative gene responsible for DFNA17, an autosomal dominant nonsyndromic form of hereditary hearing impairment. MYH9, a conventional nonmuscle myosin, joins the growing list of myosins associated with hearing loss. In the DFNA17 family, a G to A transition at nucleotide 2114 changes codon 705 from an invariant arginine (R) to histidine (H), R705H, within a highly conserved SHI linker region. The co-segregation of the mutant MYH9 with nonsyndromic hearing impairment illustrates a biologically significant role for MYH9 in hearing and an organ-specific pathology associated with the mutant allele.The objective of the proposed research is to understand the role of MYH9 and its mutant allele MYH9R7O5H in hearing and its dysfunction. We will test the hypothesis that MYH9 is essential for normal hearing and that the mutant allele MYH9R7O5H leads to myosin dysfunction and auditory impairment. Initially, we will assess the importance of MYH9 to hearing in humans. Individuals with hearing loss of unknown genetic etiology will be screened for alterations in the MYH9 gene to determine the contribution of MYH9 mutations in nonsyndromic hearing impairment. The discovery of additional mutations will facilitate genotype-phenotype correlation and determining the contribution of MYH9 to hearing loss in general. Secondly, we will assess the role of MYH9 in inner ear development and hearing. This will be carried out by characterizing the expression pattern of Myh9 in the developing and an adult mouse inner ear and determining the effects of its absence. We will use the XA1 36 ES cell line carrying a marker gene insertion into its Myh9 allele to generate mice transgenic for the Myh9 null allele. Thirdly, we will assess the effect of the MYH9R7O5H mutation on myosin function in vitro and in vivo. MYH9R7O5H will be characterized in vitro for its ATPase activity, actin-dependent motility and the effect of its expression in cultured cell lines in which biological role of MYH9 has been established. Generating a mouse model of DFNA1 7 through targeted or random germline introduction of Myh9R7O5H allele will assess the effect of MYH9R7O5H in vivo. In summary, the proposed research will lead to elucidation of the role of MYH9 in hearing and deafness.